Flow cytometry is a powerful analytical technique used to measure many properties and investigate many parameters of cells, engineered microspheres, microscopic organisms, and particles in solution for applications that range from biomedical diagnostics to monitoring of environmental states.1-4 In conventional flow cytometry, a suspension to be analyzed is focused into a single, fine stream using hydrodynamic focusing by a high-pressure sheath fluid that constricts the sample. In combination with a tightly focused laser this precise positioning creates a small interrogation volume that is analyzed via high numerical aperture optics. The collected light is typically distributed via conventional optics to several photodetectors to provide multiple parameters of fluorescence and scatter for each cell or particle. Conventional flow cytometers can analyze cells at rates as high as 50,000 cells/sec5 but for many applications that have clinical relevance such as the detection of circulating tumor cells (CTC) that are present in blood at levels as low as a 100 cells per milliliter of blood, the current analysis speed of conventional flow cytometers is inadequate.6 Accurate and simple detection of CTCs in blood samples is becoming a highly sought after diagnostic for cancer detection and treatment monitoring applications.7 Additional clinical applications, such as detection of fetal cells in maternal blood for prenatal diagnosis8-10 and endothelial progenitor cells that have roles in cancer and cardiovascular disease,11-13 which require analysis of billions of cells in regular basis, could also benefit from improved analysis rates.
While the need for high analysis rates to support rare event detection applications has been recognized, higher analysis rates are limited in conventional flow cytometry by several parameters that include: detector sensitivity, data acquisition electronics, system pressure, and coincidence rates of particles within the analysis point of the flow cytometer. Detector sensitivity limits the rate of analysis since increasing particle analysis rates typically results in shorter interrogation times, which has led to the use of highly sensitive and fast detectors such as photomultiplier tubes or avalanche photodiodes. Extremely short transit times also pose a challenge for data acquisition where digitization of signals from multiple detectors with 14-bit analog-to-digital converters (ADCs) running at nearly 100-MHz greatly increase the cost and complexity of the system. Additionally, as the linear velocity of the sample stream is proportional to the square root of the sample delivery pressure, conventional systems require greater than 1 MPa to drive samples at 10 m/s.14 Thus, mechanical limitations of the flow cell as well as deleterious effects on cells can restrict the pressure being applied to the system. Finally, the maximum analysis rate is also determined by the stochastic nature of cellular arrival at the interrogation volume, which limits the concentrations of cells that can be used without causing an intolerable number of coincidences following the Poisson distribution of particle arrival times. Due to a combination of the above limitations, a conventional single stream flow cytometer is roughly limited to an analysis rate of 50,000 cells per second.5 
To achieve higher analysis rates it has become necessary to explore the use of parallel analysis streams. For numerous reasons, the use of multiple independent channels (regardless of shape) with independent focusing elements would likely be unacceptably complex. Accordingly there is a need for mechanisms to produce multiple stream lines in a single channel. To a modest extent this approach has been successfully achieved via the use of four hydrodyamically focused simultaneous stream lines, which enables analysis and sorting at rates reported to be greater than 250,000 cells per second.15 However, alternative approaches to particle focusing such as acoustic, inertial, and dielectrophoretic positioning have the advantage of concentrating particles to precise positions without the concurrent acceleration imparted by hydrodynamic focusing.16-25 These approaches offer the potential to create many parallel streams with modest linear velocities, which might greatly simplify the creation of highly parallel flow cytometers with even higher analysis rates and greatly reduced system cost and complexity. Furthermore, these techniques do not require a sheath flow, thus fluid consumption and hazardous waste output is minimized.
While the concept of highly parallel flow streams has been explored in part through the use of highly parallel inertial focusing channels,19 there may be distinct advantages to the use of acoustic focusing. Acoustic focusing employs an ultrasonic standing wave to position particles suspended in a fluid-filled cavity, via a time-averaged drift force that transports them to a nodal or anti-nodal position.26 If particles are more compressible and less dense than the surrounding fluid then they are driven to the pressure antinodes of a standing wave, while if they are denser and less compressible than the surrounding fluid they are driven to the pressure nodes.17 
Acoustic focusing cells in a variety of forms have been developed for a many applications.27-33 In planar standing waves, particles are typically regularly spaced at half-wavelength intervals parallel to the direction of acoustic wave propagation.16,19,34 However, the use of cylindrical transducing elements can drive standing waves with a two dimensional structure and an axially positioned focusing node in the center of the capillary, which is analogous to how a traditional hydrodynamic focusing flow cell functions.35 As the optimal resonant frequency of all acoustic cells varies based on the viscosity, density, and temperature of the sample, this approach requires active control to maintain optimal focusing.35,36 